1. Field of Invention
The present invention relates to a method and apparatus for determining the drag factor that exists when two surfaces are in contact and move relative to each other.
2. Description of Prior Art
When gravity supplies the normal force, the term "drag factor" is synonymous with the coefficient of friction if the two surfaces are horizontal. The drag factor will differ from the coefficient of friction when measured on inclines as the drag factor alone measures the combined influence of the coefficient of friction and the slope of the incline. By measuring on an incline, the results will reflect the effect of the incline in overcoming the friction forces between the two surfaces. This could be important in accident reconstruction because it will provide a direct measure of the actual drag factor at work, a slope compensated static or dynamic drag factor. Currently, it is common in accident reconstruction for a coefficient of friction to be used in conjunction with a correction grade drag factor in order to estimate the drag factor.
There are several known methods to measure the coefficient of friction or the drag factor. A widely used device for the measurement of the drag factor is the Horizontal Pull Slipmeter, hereinafter referred to by the acronym HPS. The HPS was developed by C. H. Irvihe of Liberty Mutual Insurance Company of Hopkinton, Mass. It is described in ASTM Method F:609-79. The HPS consists of two basic elements, a drag sled mounted on slider feet and equipped with a force gauge and a means for pulling the sled at a controlled rate of speed across the surface being tested. A drawback to this device is that the alignment of the pulling mechanism is not controlled. This may introduce an additional variable which could affect measurement accuracy. In addition, this type of device is susceptible to drag sled bounce or hop. Drag sled hop occurs when the frictional force between the drag sled and the test surface is abruptly overcome by the increasing pulling force on the drag sled. Depending on test conditions, the drag sled hop can increase in a nonrecoverable manner rendering the test results invalid. In addition, a separate drag sled is required for each different material to be tested. Thus, this type of device does not allow for easy testing of different materials. Another device, U.S. Pat. No. 4,895,015 issued to English in January 1990, is an improvement to the horizontal pull slipmeter which utilizes a drag sled and a stationary pulling mechanism with a set of guide tracks. The drag sled is pulled by a motor and guided along the tracks which assure the alignment of the pull. The force required to pull the drag sled is measured thus allowing the determination of the drag factor. However, the guide rails may introduce some error if they are not correctly aligned which may cause the drag sled to bind and thus affect the accuracy of the test. Also, if the surface is not perfectly flat the sled will bind on the rails introducing an error. In addition, the friction of the drag sled with the rails will effect the test results by inflating the force needed to pull the drag sled and thus inflating the measured drag factor.
Another device, U.S. Pat. No. 4,813,266 issued to Nash in March 1989, consists of a test block connected to a microprocessor. The bottom of the test block is covered with the material whose drag factor is desired. The test block is set on the test surface and the block is given an impulse with a hammer or other object. The impulse causes the block to move along the surface. An accelerometer on the test block transmits acceleration data to the microprocessor. The acceleration data includes information on the acceleration of the block caused by the impulse and the deceleration of the block as it comes to a stop after the impulse. This acceleration data is used by the microprocessor to determine the drag factor at a specific point on the test surface per measurement. This may be a drawback in some cases where it might be desirable to know the average value over some distance.
Three known devices utilize a motor vehicle to measure the drag factor between a tire and a road surface. These devices are:
1. U.S. Pat. No. 3,893,330 issued to Shute et al in July 1975 describes a device which utilizes a special fifth wheel on a vehicle to determine drag factor. While in motion the vehicles brakes are applied. The speed of the vehicle and the distance travelled before the vehicle stops after the brakes are applied are used to determine the drag coefficient; PA1 2. U.S. Pat. No. 4,212,063 issued to Hardmark in July 1986 also uses a fifth wheel. The fifth wheel is mounted to a vehicle in such a manner that it can be lowered to come into contact with the road surface. The vertical forces acting on the measuring wheel are preset to known values. The torque acting on the fifth wheel is measured. This torsional force and the known vertical forces are used to determine the drag factor. PA1 3. U.S. Pat. No. 4,779,447 issued to Rath in October 1988, delivers a different amount of brake pressure to one wheel of a vehicle than to another. When the vehicle is being driven and the brakes are applied the rotational speed of the wheels is compared to determine wheel slip. The wheel slip, brake torque and axle load are then used to determine the drag factor. PA1 (a) to provide a device that will measure static and dynamic drag factors and thus provide the user with a slope compensated drag factor; PA1 (b) to provide a device that alerts the user if the pull test is improperly conducted; PA1 (c) to provide a device that will allow for convenient and rapid testing of different materials; PA1 (d) to provide a device that is easy to use; PA1 (e) to provide a device that is easily portable; PA1 (f) to provide a device that is relatively simple to manufacture and relatively inexpensive; PA1 (g) to provide a device that resists friction test block "hop";
An inherent drawback with these devices is that in motor vehicle accident reconstruction the test vehicle must travel along the same path as the vehicle in the accident. In some cases this may be hazardous. For example, if an accident occurs at a sharp turn in the road or near a large dropoff, it may not be safe to drive the test vehicle along the same path as that taken by the vehicle involved in the accident. Also, in order to obtain an accurate reading the test vehicle must a similar breaking efficiency and tire rubber compound as the vehicle that was involved in the accident. This may be difficult to obtain in some cases. For example, if the vehicle involved in the accident was determined to have a leaking wheel cylinder, it would be difficult to duplicate this accurately on the test vehicle. Another drawback to these devices is that they are not easily portable. For example, if the accident reconstruction professional must travel a long distance to reach an accident site the test vehicle must be driven or otherwise transported to the site or the fifth wheel and other equipment must be transported to the site and installed on another vehicle at the accident site.
Another device, U.S. Pat. No. 4,594,878 issued to Abe et al in June 1986, uses a rotating tire which is lowered into contact with the surface to be measured. The change in tire rotating speed is used to determine the drag factor. This differs from those devices that utilize a fifth wheel in that it is not mounted on a vehicle. Since the measuring apparatus is stationary it will not take into account the effect of an incline and thus will measure only the friction coefficient and will not provide a measure of the drag factor. In addition, this device will provide a measure of the coefficient of friction at only one point per measurement when in some cases it might be desirable to know the average value over some distance.